1. Field of the Invention
This invention relates to vacuum interrupters and, more particularly, the design and arrangement of slotted disk-shaped contacts for high-current vacuum interrupters.
2. Description of the Prior Art
Vacuum interrupters are typically used, for instance, to reliably interrupt medium to high voltage ac currents of several thousands of amperes or more. They generally include a vacuum envelope enclosing a pair of facing contact electrodes that are relatively movable between a closed circuit position and an open circuit position. Each contact is connected to a current-carrying terminal post extending outside the vacuum envelope. Surrounding the contacts within the envelope is a vapor condensing shield aligned concentrically with the contacts and terminal posts.
When the contacts are moved apart from the closed circuit position to the open circuit position, arcing of the current between the contacts occurs before the current is interrupted. The arcing can seriously damage the contacts, reducing the useful life of an interrupter. Metal from the contacts that is vaporized by the arc condenses back onto the contacts and also on the vapor shield, protecting the insulating vacuum envelope from accumulating deposits of metal.
The designs of practical commercial high-current vacuum interrupters have evolved over the past thirty years into two main types of contact arrangements, discussed in an article authored by one of the present inventors. P. G. Slade, The Vacuum Interrupter Contact, IEEE TRANS. ON COMPONENTS, HYBRIDS, AND MFG. TECH., Vol. CMHT-7, No. 1, p. 25-32, March 1984, included in this specification by reference. Each type produces a magnetic field which helps to control the initially columnar arc and promote its transition to a diffuse mode. In a first type, an axial magnetic field is generated in the contact region that forces the high-current arc to rapidly become diffuse and continuously distributed within the contact gap. In a second type, a magnetic field is impressed perpendicular to the arc column in a direction which forces the arc to move rapidly around the circular periphery of the contact surface. This can be accomplished using slotted-cup or spiral-shaped arm contacts, wherein the magnetic field is self generated by the ac current. In some contact designs of this type, the confronting contacts each have a four-slot arrangement that mirrors the other contact, rotated by about 45.degree., as disclosed in U.S. Pat. No. 3,809,836 to Crouch.
During high-current arcing with present spiral-arm contact designs and a metal shield, it has been observed that the Lorentz force drives the arc along the periphery of the contacts. The arc column is frequently perturbed by a slot located between two arms and expands outward to burn from one contact to the shield and back to the other contact. The arc takes on a more diffuse appearance and continues its azimuthal motion in this condition until it reverts back to a column in the gap between the contacts. However, adverse effects can reduce the probability of successful current interruption. The contact slots can be bridged due to melting of the spiral-shaped arms, or petals of the contact, especially when a columnar arc becomes stationary at the tip of a contact petal. The longer a columnar arc remains stationary at the tip, the greater is the melting of the contact at that spot. The columnar arc can become anchored at a position between the contacts at a slot, especially where a slot has become bridged due to melting of the contacts. The arc becomes even more firmly anchored, severely eroding the arc roots, and sometimes leading to interruption failure. Adjacent to this position, the shield can suffer heavy melting.
The radial component of the force on the arc column can force the arc to attach to the nearby vapor shield which then becomes, in effect, a third electrode. This occurs most frequently when the arc is passing over or is fixed at a slot at the contact periphery. With judicious choice of shield material and thickness, the shield can withstand the energy of the arc. For a description of an appropriate type of vapor shield, the reader is referred to U.S. Pat. No. 4,553,007 to Wayland, assigned to the assignee of the present invention, and included in this specification by reference. The striking of the arc upon the vapor shield can be advantageous, because once the arc is attached to the shield the arc spreads out, reducing the energy deposited at the contact petal tip. A fixed arc can then resume its tendency toward circumferential motion. The shield does not sustain serious damage where the arc attaches unless the nearby contact slot has already been bridged.
There is therefore a need for a spiral-arm vacuum interrupter having a contact arrangement that delays bridging of the slots until higher currents, and that encourages the participation of the vapor shield as a third electrode in arcing between the contacts.